Deep in the ocean, a fish called the barreleye has evolved one of nature's strangest heads: a completely transparent dome of bone and tissue, with two barrel-shaped eyes that point upward like searchlights. These remarkable eyes take up most of the space inside that see-through skull, giving the barreleye its name and its uncanny appearance.

The barreleye lives in the twilight zone of the deep sea, where sunlight barely penetrates. It hunts by looking upward, where its tubular eyes are perfectly designed to spot the faint silhouettes of prey swimming above it against whatever dim light filters down from the surface. But the barreleye is not locked into this upward gaze. Its eyes can rotate and point forward as well, giving it flexibility to track and pursue targets when the hunting opportunity arises.

These small fish belong to the family Opisthoproctidae and live throughout tropical and temperate deep waters in the Atlantic, Pacific, and Indian Oceans. Their transparent head isn't just a strange adaptation for show, it reduces drag in the water and keeps the fish streamlined while its bulging barrel eyes gather every photon of available light. The barreleye's combination of an invisible skull and powerful upward-gazing vision makes it one of the ocean's most effective hunters of the deep, perfectly adapted to find meals in waters where most animals would see only darkness.